Magnetron having external choke structure

ABSTRACT

A magnetron comprising an air-tight vacuum envelope including a cylindrical anode having a plurality of vanes, a cathode for thermionic emission arranged on the central axis of the anode and magnets for applying magnetic field to the working space between the anode and cathode. The magnetron is further provided with a choke structure having an opening in the vicinity of the input terminal of the air-tight vacuum envelope, the choke structure being arranged outside of the envelope and with its effective length almost one fourth of the wavelength of microwave energy generated by the magnetron.

O Unlted States Patent [1 1 [111 3,846,667 Hisada et al. Nov. 5, 1974MAGNETRON HAVING EXTERNAL CHOKE 3,551,735 12/1970 Staats 315/3953STRUCTURE 3,584,255 6/1971 Staats INS/39.53

[75] Inventors: gi 3 2; 223; @225 Harada Primary Examiner-James W.Lawrence Assistant Examiner-Saxfield Chatmon, Jr. Assigneel Hitachi, -sTokyo, Japan Attorney, Agent, or Firm-Craig & Antonelli [22] Filed: June25, 1973 211 Appl. No.: 372,903 [57] ABSTRACT A magnetron comprising anair-tight vacuum envelope [30] Foreign Application Priority Dataincluding a cylindrical anode having a plurality of J 30 1972 J 47 76597vanes, a cathode for thermionic emission arranged on une apan thecentral axis of the anode and magnets for applying magnetic field to theworking space between the 315/3953 315/ anode and cathode. The magnetronis further pro- 58] Fieid 39 51 vided with a choke structure having anopening in the vicinity of the input terminal of the air-tight vacuumenvelope, the choke structure being arranged outside [56] ReferencesCned of the envelope and with its effective length almost UNITED STATESPATENTS one fourth of the wavelength of microwave energy 3,315,1214/1967 Staats 315/3953 X generated by the magnetron. 3,377,562 4/1968Staats INS/39.53 3,543,082 11/1970 Boehm 3l5/39.53 X 3 Claims, 3 DrawingFigures PAIENIEIJNUV 51914 $846166! sum 1 or 3 FIG.

PRIOR ART PATENTEDHBV 51974 3.846667 SHEET 3!!! 3 MAGNETRON HAVINGEXTERNAL CHOKE STRUCTURE The present invention relates to a magnetron ormore in particular to an improved input structure of the magnetron.

FIG. 1 is a sectional view showing the essential parts of an example ofthe conventional magnetrons,

FIG. 2 is a sectional view showing the essential parts of an embodimentof the present invention; and

FIG. 3 is a sectional view showing the essential parts of anotherembodiment of the present invention.

The magnetron which generates microwave energy effectively is used forthe microwave oven, defreezer and the like and there are variousconstructions suggested therefor.

The sectional view of FIG. 1 shows an example of the magnetron ofinner-magnet type in general use. In the figure, reference numeral 1shows an anode cylinder made of oxygen free copper, numeral 2 aplurality of vanes fixed in such a manner as to divide equally the innerwall of the anode cylinder 1, and numeral 3 an anode comprising theanode cylinder 1 and vanes 2. Reference numerals 4 and 5 show conicalmagnets fixed on the sides of the anode cylinder 1 and numeral 6 acup-shaped input sealing metal having a bottom portion 6a and a throughhole 6b and fixed on the magnet 4. Numeral 7 shows an input insulatingmaterial arranged in abutment with the bottom portion 6a of the sealingmetal 6 and hermetically fixed on the sealing metal 6 at a connection60. Reference numeral 8 shows a spiral cathode of thoriated tungstenwire or the like for thermionic emission arranged coaxially with theanode cylinder 1 within the space defined by the edges of the vanes 2.The edges of the vanes 2 are in predetermined spaced relationship withthe spiral cathode 8 thereby to form a working space 9. Referencenumerals l0 and 11 show end shields arranged on the ends of the spiralcathode 8 for preventing the diffusion of electrons in the direction ofthe axis of the tube, numerals 12a and 12b side supports each with anend passed through the through hole 6b of the sealing metal 6 and fixedwithin the insulating material 7, and numeral 12c a center supportsimilar to the side supports 12a and 12b through which working currentis supplied, the other ends of the supports 12a, 12b and 12c being fixedon the spiral cathode for thermionic emission. Reference numerals 13a,13b and 13c show sealing members for hermetically fixing the supports12a, 12b and 12c, numeral 14 a radiator for cooling the anode 3, numeral15 an output sealing member fixed on the magnet 5 numeral 16 acylindrical output insulator fixed on the sealing member 15, numeral 17an output terminal fixed on the insulator 16, and numeral 18 a metal rodwith the ends thereof connected to one of the vanes 2 of the anode 3 andoutput terminal 17 respectively, so that microwave output generated inthe anode 3 is transmitted through metal rod 18 to the output terminal17 from which it is emitted outside. Reference numeral 19 shows amagnetic device comprising a permanent magnet, numerals 20 and 21 yokesfor introducing to the working space the magnetic field generated by themagnetic device 19, numerals 22 and 23 inductors connected to thesupports 12a and 12c, numerals 25 and 26 connectors for connecting thesupports 12a and 12c to the inductors 22 and 23, and numeral 24 acapacitor connected to the inductors 22 and 23, the inductors 22 and 23and the capacitor 24 functioning as a filter to erase noise. Numerals 27and 28 show shield cases for shielding noise waves radiated from theinductors 22 and 23, and numeral 29 a cylindrical stepped metal memberwith a flange 29a at one end thereof which is fixed on the shield casewith a plurality of screws 30, while the other end of the metal memberis forcibly fitted on the input sealing metal member 6 to prevent thenoise wave from being radiated outside through the magnetic device.Reference numeral 31 shows an insulating tube for preventing dielectricbreakdown between the supports 12a to 12, sealing members to 13c orconnectors 25 and 26 and shield cases 27 and 28, screws 30 or thecylindrical stepped metal member 29. As will be seen from the abovedescription, the air-tight vacuum envelope comprises various componentmembers including the supports 12a to 12c, input insulator 7, inputsealing metal member 6, magnets 4 and 5, anode 3, output sealing member15, output insulator 16 and output terminal 17.

In the magnetron with the above-described construction, most of themicrowave energy is utilized by being taken outside through the outputterminal 17, while small part of the microwave energy is coupled to thespiral cathode 8 and adversely affects the inductors 22 and 23 throughthe supports 12a and 12b, with the result that the inductors 22 and 23are heated to abnormally high temperatures, thereby breaking the same inan extreme case. Also, in view of the fact that the input insulator 7positioned between the input sealing metal member 6 and the sealingmembers 13b and 13c has an impedance Za to high-frequency current andthe shield case 28 has a certain impedance Zb, that part of microwaveenergy which has been transferred to the supports 12b and 12c isreflected for lack of the matching of impedances and the reflectedmicrowave energy generates an abnormal standing wave in the spiralcathode 8, so that the spiral cathode for thermionic emission isabnormally heated, resulting in unstable electrical characteristics andshort life of the magnetron.

These disadvantages depend to a large measure on the shape of the shieldcase 28 and the shape and positions of the inductors 22 and 23, and inspite of the amount of time and labor consumed for design work todetermine the shape and position of the shield case 28 and inductors 22and 23 and the position of the cylindrical stepped metal member 29 ofthe conventional magnetrons, it is very difficult to meet therequirements for elimination of the above-mentioned disadvantages.

An object of the presentinvention is to provide a magnetron with a longuseful life.

Another object of the invention is to provide a magnetron with stableelectrical characteristics.

According to the invention, there is provided a magnetron comprising anair-tight vacuum envelope, a cy lindrical anode including a plurality ofvanes, a cathode arranged on the central axis of the anode forthermionic emission, an input side insulator, a plurality of magnets forapplying magnetic field to the working space between the anode andcathode, a plurality of supports for supporting the cathode through theinput side insulator, working current being supplied through thesupports, an input side sealing metal member interposed between theanode and the input side insulator, the input side sealing metal memberbeing hermetically attached to the input side insulator, an output sidesealing member, an output side insulator, and an output terminal, theair-tight vacuum envelope being defined by the anode, the magnets, theinput side insulator, the input side sealing member, the output sidesealing member, the outputside insulator and the output terminal,wherein the magnetron further comprises a choke structure having anopening in the vicinity of the hermetic connection between the inputside insulator and the input side sealing metal member, the chokestructure having an effective length approximately one fourth of thewavelength of microwave energy oscillated by the magnetron and beingarranged on the outside of the air-tight vacuum envelope.

The inventors have developed an improved magnetron which obviates thedisadvantages of the prior art magnetron on the basis of technicalconcept that it is possible to increase greatly the transmissionattenuation and enlarge considerably the impedance as viewed from theopening to the choke by providing the choke of a wave-guide or a coaxialcable with an inner conductor having the effective length approximatelyone fourth of the wavelength involved.

The present invention will be explained in detail below with referenceto the accompanying drawings.

A sectional view of an embodiment of the invention is shown in FIG. 2,in which the same reference numerals are attached to similar componentelements as in FIG. 1. In FIG. 2, reference numeral 32 shows a metalcylinder with a flange which is arranged on the outside of the air-tightvacuum envelope of the magnetron and fixed on the shield case 28 withthe screw 30 through the flange 32a and the flange 29a of thecylindrical stepped metal member 29 which is closely attached to theflange 32a. The cylindrical portion 32b is arranged coaxially with thecylindrical stepped metal member 29 and the sealing metal member 6 andextends toward the connection 60 between the input sealing metal member6 and the input insulator 7 while leaving predetermined space 33. Theconnection 60, the cylindrical stepped metal member 29 and the flangedcylinder 32 constitute a choke structure with the space 33 as an openingin such a manner that the effective length lof the choke is set atapproximately one fourth of the length of the microwave generated by themagnetron, by appropriately determining the length of the cylindricalportion 32b of the flanged cylinder 32 and the length of the largediameter cylinder portion 29b of the metal member 29.

The above-mentioned arrangement permits impedance Zc of the choke asviewed from the opening of the space 33 to be increased greatly in spiteof the three supports 12a to 120 being arranged internally, and as aresult both the impedance Za due to the input insulator 7 and impedanceZb within the shield case 28 are negligible. For this reason, thestanding wave occurring in the cathode 8 depends on the position of theopening of the choke structure and impedance Zc thereof, so that bylocating an end of the opening of the choke structure at the connection6c of the sealing metal member as in the above-described embodiment, itis possible not only to minimize the standing wave in the cathode butalso to maximize the tolerance of effective length of the choke. This,together with the employment of such choke structure, contributes to theadvantage of greatly increased transmission attenuation, resulting ingreat attenuation of micro-wave energy transmitted into the shield case28 thereby to prevent abnormal temperature increase of the inductors 22and 23.

Further, the insulating tube 31 is interposed between the flangedcylinder 32 and the insulator 7 in such a manner as to cover the space33, so that the effect of the dielectric constant of the insulating tube31 increases the degree of coupling of the choke structure, resulting inan increased efficiency of the choke.

Another embodiment of the invention comprising a magnetron of innermagnet type is illustrated in F IG.3.

In this drawing, reference numeral 42 shows a cupshaped stepped metalmember with an end 42a of the large-diameter side wall thereof fixed onan appropriate position on the shield case 48, while the smalldiameterbottom portion 42b is arranged around the input insulator 47 in such amanner as to provide predetermined space 43 between the bottom portion42b and the connection 46a of the input sealing metal member 46. In thisway, the cup-shaped metal member 42, part of the shield case 48 and theconnection 46a make up a choke structure with the space 43 as anopening. It is needless to say that the size of the cupshaped metalmember 42 as well as the distance from the connection between the shieldcase 48 and the sealing metal member 46 to the connection of the shieldcase 48 and the part 42a of the large-diameter portion of the cup-shapedmetal member 42 is so selected that the effective length I of the chokeis approximately one fourth of the wavelength of the microwave generatedby the magnetron. Reference numeral 44 shows a capacitor, numeral 49amagnetic device and numeral'50 a magnetron proper.

As will be noted from the above description, the mag-- netron accordingto the present invention is characterized by a choke structure providedoutside of the airtight vacuum envelope of the magnetron and with itsopening situated in the vicinity of the input insulator and theconnection of the sealing metal member'so that the effective length ofthe choke structure is approximately one fourth of the wavelength of themicro-wave energy generated by the megnetron, resulting in a highimpedance of the choke structure. For this reason, both the impedancedue to the insulator and the impedance within the shield case arenegligible and the fact that microwave energy transmitted into theshield case is greatly attenuated permits the prevention of the temperature of the inductors from being increased to an abnormally highlevel. Also, since the opening of the choke is situated in the vicinityof the periphery of the input insulator and the sealing metal member,the standing wave generated in the cathode for thermionic emission isminimized while the tolerance of the choke effective length l ismaximized. Further, in view of the fact that the choke structure islocated outside of the air-tight vacuum envelope of the magnetron, theassembly work of the magnetron is facilitated and an abnormaltemperature increase which is often seen in the prior art magnetroncontaining the choke inside thereof is prevented. Furthermore, theprovision of the choke structure outside of the air-tight vacuumenvelope makes possible impedance adjustment even after the completionof magnetron in accordance with the characteristics requirementsthereof. Still another advantage of the magnetron according to theinvention resides in the fact that the shield case, inductors and otherequipment are easily designed because it is only their breakdownvoltages that require to be taken into consideration because of actuallyneglecting the effect of microwave energy upon such equipment.

What we claim is:

l. A magnetron comprising an air-tight vacuum envelope, a cylindricalanode including a plurality of vanes, a cathode arranged on the centralaxis of said anode for thermionic emission, an input side insulator, aplurality of magnets for applying a magnetic field to the working spacebetween said anode and cathode, a plurality of supports for supportingsaid cathode through said input side insulator, means for supplyingworking current through said supports to said cathode, an input sidesealing metal member interposed between said anode and said input sideinsulator, said input side sealing metal member being hermeticallyclosely attached to said input side insulator, an output side sealingmember, an output side insulator, and an output terminal; said air-tightvacuum envelope being defined by said anode, said magnets, said inputside insulator, said input side sealing member, said output side sealingmember, said output side insulator and said output ter minal, whereinsaid magnetron further comprises a choke structure having an opening inthe vicinity of the connection between said input side insulator andsaid input side sealing metal member, said choke structure having aneffective length approximately one fourth of the wavelength of microwaveenergy oscillated by said magnetron and being arranged on the outside ofsaid air-tight vacuum envelope.

2. A magnetron according to claim 1, further comprising an insulatingtube fitted around the periphery of said input side insulator andextending from said connection between said input side insulator andsaid input side sealing metal member, said choke structure comprising afirst flanged metal cylinder and a second flanged stepped metalcylinder, said first flanged metal cylinder having a cylindrical portionfitted around the periphery of said insulating tube and extending towardsaid connection, said cylindrical portion maintaining predeterminedspaced relationship with said connection, said second flanged steppedmetal cylinder being arranged cocentrically with said first flangedmetal cylinder and having a small-diameter cylinder portion forciblyfitted on said input side sealing metal member, said second flangedstepped metal cylinder further having a large-diameter cylinder portionwith a diameter larger than the outer diameter of said cylindricalportion of said first metal cylinder, the flange of said second flangedstepped metal cylinder being closely attached to the flange of saidfirst flanged metal cylinder.

3. A magnetron according to claim 1, further comprising a shield casecovering the exterior component elements on the input side of saidair-tight cacuum envelope, said shield case having part of the bottomthereof comprising part of said input side sealing metal member, saidshield case containing a stepped cupshaped metal member concentricallyarranged with said input side insulator and input side sealing metalmember, said stepped cup-shaped metal member having a bottom with a holethrough which said input side insulator is passed, said steppedcup-shaped metal member having a large-diameter portion with its innerbottom surface spaced from the inner bottom surface of said shield case,said stepped cup-shaped metal member having a small-diameter portionwith its inner bottom surface spaced from the upper surface of saidinput side sealing metal member, the wall end of said large-diameterportion of said stepped cup-shaped metal member being fixed on the innerbottom surface of said shield case; said choke structure comprising saidstepped cup-shaped metal member, part of said shield case and part ofsaid input side sealing metal member.

1. A magnetron comprising an air-tight vacuum envelope, a cylindricalanode including a plurality of vanes, a cathode arranged on the centralaxis of said anode for thermionic emission, an input side insulator, aplurality of magnets for applying a magnetic field to the working spacebetween said anode and cathode, a plurality of supports for supportingsaid cathode through said input side insulator, means for supplyingworking current through said supports to said cathode, an input sidesealing metal member interposed betweeN said anode and said input sideinsulator, said input side sealing metal member being hermeticallyclosely attached to said input side insulator, an output side sealingmember, an output side insulator, and an output terminal; said air-tightvacuum envelope being defined by said anode, said magnets, said inputside insulator, said input side sealing member, said output side sealingmember, said output side insulator and said output terminal, whereinsaid magnetron further comprises a choke structure having an opening inthe vicinity of the connection between said input side insulator andsaid input side sealing metal member, said choke structure having aneffective length approximately one fourth of the wavelength of microwaveenergy oscillated by said magnetron and being arranged on the outside ofsaid air-tight vacuum envelope.
 2. A magnetron according to claim 1,further comprising an insulating tube fitted around the periphery ofsaid input side insulator and extending from said connection betweensaid input side insulator and said input side sealing metal member, saidchoke structure comprising a first flanged metal cylinder and a secondflanged stepped metal cylinder, said first flanged metal cylinder havinga cylindrical portion fitted around the periphery of said insulatingtube and extending toward said connection, said cylindrical portionmaintaining predetermined spaced relationship with said connection, saidsecond flanged stepped metal cylinder being arranged cocentrically withsaid first flanged metal cylinder and having a small-diameter cylinderportion forcibly fitted on said input side sealing metal member, saidsecond flanged stepped metal cylinder further having a large-diametercylinder portion with a diameter larger than the outer diameter of saidcylindrical portion of said first metal cylinder, the flange of saidsecond flanged stepped metal cylinder being closely attached to theflange of said first flanged metal cylinder.
 3. A magnetron according toclaim 1, further comprising a shield case covering the exteriorcomponent elements on the input side of said air-tight cacuum envelope,said shield case having part of the bottom thereof comprising part ofsaid input side sealing metal member, said shield case containing astepped cup-shaped metal member concentrically arranged with said inputside insulator and input side sealing metal member, said steppedcup-shaped metal member having a bottom with a hole through which saidinput side insulator is passed, said stepped cup-shaped metal memberhaving a large-diameter portion with its inner bottom surface spacedfrom the inner bottom surface of said shield case, said steppedcup-shaped metal member having a small-diameter portion with its innerbottom surface spaced from the upper surface of said input side sealingmetal member, the wall end of said large-diameter portion of saidstepped cup-shaped metal member being fixed on the inner bottom surfaceof said shield case; said choke structure comprising said steppedcup-shaped metal member, part of said shield case and part of said inputside sealing metal member.